Friday, September 26, 2008

Lesson 12: Cross Bearing Fix

Using NOAA Chart 12221 TR, Variation 9W, and the deviation table from Lesson 10 -Relative Bearings, Lines of Position and Cross Bearing FixYou may determine the position of your boat by many methods of piloting. The line of position is common to all methods. For example, if you observe a charted standpipe and a charted flag pole in range (lined up), you are somewhere on the line drawn from the standpipe to your boat through the flagstaff. This line is called a range line and is a line of position (LOP). Although a single visual observation can provide a line of position, it does not establish a position. You are located somewhere along this LOP. If an LOP is obtained by magnetic compass, the compass bearing must be corrected to true degrees by apply variation and deviation for the compass heading at the time the bearing was obtained.

Remember a single line of bearing gives you a LOP. Your boat is somewhere along that LOP. You also know that you cannot accurately fix your position by a single LOP. You must plot two (2) or more intersecting LOP’s to obtain a Cross Bearing Fix. The greater number of lines of position intersecting at the same point the greater the confidence in the fix. For a fix to be most accurate, LOP’s must be derived from simultaneous observations or bearings time corrected. In the normal practice of small-boat navigation, you may take two (2) or more bearings, one after the other, and these are considered to meet accuracy requirements.

Bearings can be taken by sighting across a compass, using a hand bearing compass, relative bearings or by RADAR. The direction to the object sighted is recorded. When using cross bearings the fix is obtained by taking bearings on two (2) well defined – charted objects and plotting the observed bearings onto a chart. A more accurate fix may be obtained by taking a third bearing on another well defined object. There should be a separation of at least sixty (60) degrees between lines of position.

Need to Know

A line of position or relative bearing is where the observer and sighted object is assumed to be located somewhere in the same visual “range”.

Two intersecting lines of position taken on two separate sighted objects (objects with known, charted positions) can provide the position of the observer.

Three lines of position on three separate objects (all with known positions), provide a far more accurate fix than using two lines of position.

Only a ship’s heading has a deviation value, relative bearings obtained from lines of position never have deviation values.

Relative bearings are expressed in relation to the ship’s steering compass.

Always work relative bearings up from magnetic steering compass readings to true direction before plotting on a chart.

Always plot bearings values in the correct direction and in true degrees “back” towards the respective sighted object used for that specific bearing or line of position.

Learning Exercise: Cross Bearing FixIn the following problem set, you will be given the vessel’s heading per steering compass plus three relative bearings. Find Lat/Lon using this information. Remember that deviation is taken from the ship’s heading (SH), not relative bearing(s).

Remember !

Using only the ship’s heading to determine deviation value and use the same value for each “D” in table below. Remember, remember, and remember….bearings never have deviation values! Bearings never deviate, only a ship’s head deviates.

Work each relative bearing reading up to True degrees (- W + E).

On the chart, plot each reading in True degrees, in the correct direction, towards its respective sighted object.

At the point which the lines of position intersect indicate the vessel’s position at the time when the compass bearings were taken.

Drop the pencil in the geometric center of the triangle or cocked hat.

Determine LATITUDE AND LONGITUDE

Learning Exercise:

1. You are on a course of 027º PMC when you take the following bearings PMC: